Polarimeters are devices used to measure the polarisation state of electromagnetic waves (i.e. electromagnetic illumination) and are used in various fields such as organic and biological microscopy, crystallography and astronomy.
Many polarimeters work by passing electromagnetic rays of unknown, or partially unknown polarisation state through an optical assembly the polarisation co-ordinate system of which is known. That is, the optical assembly imparts a known change in the polarisation state of electromagnetic rays that pass through it. Then, by providing a detector that measures changes in the properties of the electromagnetic rays after they have passed through the optical assembly, information about the original polarisation state of the electromagnetic rays can be calculated.
In many polarimeters, the optical assembly comprises one or more polarisation modulators. Polarisation modulators are devices the optical properties of which vary in response to the application of modulating stimuli. Electromagnetic rays that pass through a polarisation modulator are subject to a change in polarisation state that varies in accordance with the applied modulating stimuli. Polarisation modulators can be arranged so that output electromagnetic rays that have undergone modulation, vary in intensity in correspondence with the imparted polarisation changes. This variance in intensity can be readily detected by a photo-detector. As is known, by analysing the variances in the intensity of the modulated electromagnetic rays information about the polarisation state of the light can be determined.
Advanced polarimeters often employ optical phase modulators for modulating the polarisation of light. Optical phase modulators, such as photoeleastic modulators (PEMs), are often preferred because they have superior optical and modulating properties. However, such modulators typically operate at very high modulation frequencies (in the order of tens of kHz) which means that the detectors in systems including such modulators must be capable of operating at such frequencies.
This is a particular problem for imaging polarimeters. Imaging polarimeters are arranged to measure polarisation parameters of electromagnetic rays reflected, emitted or transmitted by an object (or scene) with a degree of spatial resolution. Such devices typically employ a two-dimensional photo-detector array. However, commonplace imaging devices such as digital cameras cannot operate at frame rates that are as high as typical optical phase modulator modulating frequencies. Accordingly, for imaging polarimeters at least, complex custom sensors are typically required which can be difficult to integrate into existing systems and are generally much more expensive than “off-the-shelf” digital camera type devices.
Modification of existing digital camera devices may be possible but would typically result in reduced polarimeter performance. For example, it may be possible to increase the frame rate of digital camera type devices by replacing alternate rows of detector pixels with storage devices. However, this reduces the number of working detector pixels and correspondingly the resolution of the device.
It is desirable to provide a polarimeter that can use a conventional photo detector array as a sensor. It would be particularly desirable to provide an imaging polarimeter that can use a conventional two dimensional photo detector array as a sensor.